Euan Ashley, professor of cardiovascular medicine at Stanford University, carried out some research on seven top fitness trackers to see how accurate they were.

The wrist-worn wearable devices included the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn, and Samsung Gear S2. They were pitted against gold standard medical gear.

Ashley said that while he was surprised at how good the gear was at measuring heart rate, the estimates of calorie burn was pretty pants.

“We were unpleasantly surprised at how poor the calorie estimates were for the devices -- they were really all over the map."

The team tested seven -- with 31 women and 29 men each wearing multiple devices at a time while using treadmills to walk or run, cycling on exercise bikes or simply sitting.

The results, published in the Journal of Personalised Medicine said that the most errors on energy expenditure were far greater, ranging from the lowest at 27.4 percent for the FitBit Surge to the highest error of 92.6 percent for the PulseOn device.

The errors in energy expenditure, said Ashley, could be down to a range of factors including problems with the devices’ algorithms or poor data input by users. Errors were found to vary due to factors including sex and mode of exercise [isn't sex a mode of exercise? ed]

The team says the findings have ramifications for those relying on their fitness trackers as a measure of their health.

“When you consider that people are using these estimates to make lifestyle decisions like what they are going to eat for lunch then I think that is something that is worth knowing and people should know to take these estimates with more than a pinch of salt,” Ashley said.